1. Field of the Invention
This application relates to hearing aids. More specifically, it relates to hearing aids comprising digital output stages for driving acoustic output transducers. The invention further relates to a method for driving a digital output stage of a hearing aid.
In this context, a hearing aid is defined as a small, battery-powered device, comprising a microphone, an audio processor and an acoustic output transducer, configured to be worn in or behind the ear by a hearing-impaired person. By fitting the hearing aid according to a prescription calculated from a measurement of a hearing loss of the user, the hearing aid may amplify certain frequency bands in order to compensate the hearing loss in those frequency bands. In order to provide an accurate and flexible amplification, most modern hearing aids are of the digital variety.
Contemporary digital hearing aids incorporate a digital signal processor for processing audio signals from the microphone into electrical signals suitable for driving the acoustic output transducer according to the prescription. In order to save space and improve efficiency, some digital hearing aid processors use a digital output signal to drive the acoustic output transducer directly without performing a digital-to-analog conversion of the output signal. If the digital signal is delivered to the acoustic output transducer directly as a digital bit stream with a sufficiently high frequency, the coil of the acoustic output transducer performs the duty as a low-pass filter, allowing only frequencies below e.g. 15-20 kHz to be reproduced by the acoustic output transducer. The digital output signal is preferably a pulse width modulated signal, a sigma-delta modulated signal, or a combination thereof.
An H-bridge is an electronic circuit for controlling inductive loads such as electric motors or loudspeakers. It operates by controlling the direction of a flow of current through a load connected between the output terminals of the H-bridge by opening and closing a set of electronic switches present in the H-bridge. The switches may preferably be embodied as semiconductor switching elements such as BJT transistors or MOSFET transistors. This operating principle permits a direct digital drive output stage to be employed in order to enable a suitably conditioned digital signal to drive a loudspeaker directly, thus eliminating the need for a dedicated digital-to-analog converter and at the same time reducing the power requirements for the output stage.
A sigma-delta modulator is an electronic circuit for converting a signal into a bit stream. The signal to be converted may be digital or analog, and the sigma-delta modulator is typically used in applications where a signal of a high resolution is to be converted into a signal of a lower resolution. In this context, a sigma-delta modulator is used for driving the H-bridge output stage in the hearing aid.
The diaphragm of a loudspeaker has a resting or neutral position assumed whenever no current flows through the loudspeaker coil and two extreme positions assumed whenever the maximal allowable current flows in either direction through the loudspeaker. By applying a sufficiently fast-changing bit stream from an H-bridge represented by positive and negative voltage impulses to the loudspeaker terminals, any position between the two extreme diaphragm positions of the loudspeaker may be attained. The higher the number of positive impulses in the bit stream is, the more the loudspeaker diaphragm will move towards the first extreme position, and the higher the number of negative impulses in the bit stream is, the more the loudspeaker diaphragm will move towards the second extreme position. Due to the low-pass filtering effect of the loudspeaker coil, no audible switching noise will emanate from the loudspeaker when driven in this way, provided the switching period of the bit stream is well above the reproduction frequency limit of the loudspeaker. Thus, a digital bit stream may control a loudspeaker directly.
2. The Prior Art
From EP-B1-1716723 is known a digital output stage for a hearing aid, said output stage comprising a sigma-delta converter and an H-bridge for driving an acoustic output transducer for a hearing aid. The output stage is denoted a three-state output stage because it is capable of delivering a bit stream consisting of three individual signal levels to the acoustic output transducer. In the following, these levels are denoted “+1”, “−1” and “0”, where “+1” equals the maximum positive voltage across the acoustic output transducer, “−1” equals the maximum negative voltage across the acoustic output transducer, and “0” equals no voltage. This utilizes the fact that a positive voltage pulse makes the diaphragm of the acoustic output transducer move in one direction, and a negative voltage pulse makes the diaphragm of the acoustic output transducer move in the other direction. By delivering a clocked bit stream consisting of “+1”-levels and “−1”-levels interspersed with “0”-levels as voltage pulses to the acoustic output transducer, any position deviation within the confinements of the mechanical suspension of the acoustic output transducer diaphragm may thus be obtained, as the loudspeaker coil acts as an integrator of the voltage pulses. The digital output stage of the prior art generates the “0”-level by applying a “+1”-level and a “−1”-level simultaneously to both terminals of the acoustic output transducer.
This way of generating the “0”-level for the acoustic output transducer has the advantages of being very easy to implement, as no extra components are needed to provide the “0”-level, and to save power, as the “0”-level uses no extra current and the provision of three separate levels effectively doubles the possible voltage swing across the acoustic output transducer. However, it also has some inherent drawbacks, which will be explained in greater detail in the following.
The “+1”-levels and “−1”-levels both generate differential voltages over the wires and terminals of the acoustic output transducer. This is not the case with the “0”-level. With the “0”-level, both wires carry the same voltage simultaneously, and since this is a rapidly switching voltage it radiates more common mode signal to its immediate surroundings. This radiation results in increased crosstalk to nearby surroundings, such as telecoils or wireless transmission receiver coils typically present in the hearing aid. Since this crosstalk has frequencies above 1 MHz, it does not possess a problem to the telecoil, since a telecoil is configured to convey frequencies below 8-10 kHz. The wireless receiver coil, however, suffers a very considerable reduction in signal-to-noise ratio from the capacitive interference resulting from this crosstalk phenomenon, often to a degree where reliable signal reception becomes impossible.
This capacitive interference emanates mainly from electrically exposed parts of the output circuit, primarily the wires connecting the output pads of the electronic circuit chip of the hearing aid to the input terminals of the acoustic output transducer. It is not possible to shorten these wires further for mechanical reasons, but some reduction in the capacitive coupling between these wires and sensitive electronic circuits in the vicinity may be achieved by twisting the wires and keeping them physically close together.
The voltage pulses are presented to the output transducer at a frequency of 1-2 MHz, and the resulting noise components may thus disturb the operation of electronic circuits sensitive to capacitive interference at high frequencies. In cases where the afflicted electronic equipment incorporates a wireless remote control for the hearing aid the problems caused by electromagnetic interference are exceptionally severe, as the effective operating range of the wireless remote control is limited considerably by the capacitive interference emanating from the output stage and masking the remote control signals from proper reception.
WO-A1-03/047309 discloses a digital output driver circuit for driving a loudspeaker for a mobile device such as a hearing aid or a mobile phone. The digital driver circuit comprises an input, a modulator and a three-level H-bridge and is integrated into the loudspeaker enclosure in order to shield the driver circuit from electromagnetic interference and to keep the wires connecting the driver output to the loudspeaker short. The driver circuit further comprises a feedback circuit connected to the loudspeaker for regulating the supply voltage for the driver circuit.
An output driver integrated into a loudspeaker in the way described in WO-A1-03/047309 is not interchangeable with dynamic standard loudspeakers of the kind used in hearing aids. If, for example, a hearing aid housing and circuitry may be adapted for use with a range of different loudspeakers having different impedance values, e.g. for treating different degrees of hearing loss, a loudspeaker having an integrated output driver would not be well suited for this configuration. In cases where this type of flexibility is desired, long wires between the output stage terminals of the hearing aid circuit and the terminals of the loudspeaker of the hearing aid are unavoidable. An extra set of long wires for the signal from the loudspeaker to the feedback circuit would also be required by the prior art output driver, which would further increase capacitive interference noise.